The present invention relates to weak cationic-exchange chromatographic supports. Such supports are known to provide short retention times and high efficiencies with diluted mobile phases of moderate ionic strength. This permits low detection limits. Selectivity characteristics of such materials are different from those demonstrated by strong cation-exchangers permitting certain chromatographic applications not suited for highly retentive cation-exchange phases such as sulfonated styrene-based polymers. One approach for the synthesis of weak cation-exchange materials is by direct funtionalization of a stationary phase by chemical transformation of the functional groups on a surface of the solid support. Farrall, et al., J. Org. Chem, 41, 24 (1976), 3877–3882 [Reference 1].
Another approach is by grafting or growing polymeric chains from the surface of a substrate in which weak cation-exchange sites are part of a monomeric structure. Yang, et al., Talanta 55 (2001), 1091–1096; Srinivasan et al., U.S. Pat. No. 5,792,331 (1998); Riviello et al., U.S. Pat. No. 5,968,363 (1999) [References 2–4]. A further approach is by coating the particles of a stationary phase with a pre-made polymer containing weak cation-exchange sites such as carboxylic groups. Kolla, et al., Chromatographia, 23, 7 (1987), 465–472; Nair, et al., J. Chromatography, 640 (1993), 41–48 [References 5–6]. This latter method of coating with a polymeric film permits efficient coverage of the particles and thus reduces secondary interactions between analytes and unfunctionalized regions on the surface. Also, the coating technique is a convenient way of controlling the film thickness. The substrates disclosed for the stationary phases to be coated are inorganic support materials, specifically microparticulate silica. The specific coating material is a copolymer such as poly(butadiene-maleic acid) termed “PBDMA.” The polymer in solution penetrates the silica pores and coats them. Hydrophilic instability of silica gel can cause leaching of the silicates on the substrate media in spite of the surface shield provided by a cross-linked polymeric film leading to interference with detection of the analytes. Also, the silica base supports can contaminate a suppressor membrane due to the leaching silicates.
It is an object of the invention to provide a weak cation-exchange medium with a coating of weak cation-exchange functional groups without the disadvantages of prior art stationary phases.